1. Field of the Invention
The present invention relates to an optical scanning device for use in an image forming apparatus such as a printer, facsimile, or plotter having the optical scanning device, and a multifunction product equipped with at least one such apparatus.
2. Description of the Related Art
Image forming methods using lasers as image forming units to obtain high-quality images are widely employed in electro-photographic image recording. Methods where an axial direction of a photosensitive drum in the case of electro-photography is scanned by a laser (main scanning) using a polygon mirror and the drum is then rotated (sub-scanning) to form a latent image are typical.
High-density images that are output at high speed can be obtained by employing these methods. The relationship between high-density and the output speed of images is a trade-off. It would actually be preferable to achieve both high-density and high output speed.
High-speed rotation of a polygon scanner has been considered as a way of achieving both of these purposes. However, rotating the polygon scanner at high speed causes increase in the noise and power consumption, and is detrimental to durability.
Adopting a multi-beam approach is one approach to take care of these issues and the following forms can be considered for this method:
a) A method of employing a plurality of end-emitting laser diodes (a method that has been disclosed in Japanese Patent Application Laid-open No. 2005-250319, etc.),
b) A method employing a one dimensional array of end-emitting laser diodes, and
c) A method employing a two-dimensional laser diode array.
The method of employing a plurality of end-emitting laser diodes is relatively inexpensive because it is possible to use general-purpose one-dimensional laser diodes. However, it is difficult to stably maintain a relative positional relationship between the laser diodes and coupling lenses, i.e., it is difficult to stably maintain a relative positional relationship between the beams emitted from the laser diodes. If such a relative positional relationship is not maintained, spacing of scanning lines formed on a surface being scanned by multiple beams becomes irregular leading to degradation of image quality.
In this method, it is also difficult to have an extremely large number of light sources and achieving ultra-high-density and ultra-high speeds is difficult.
It is possible to make the scanning line spacing for an end-emitting one-dimensional laser diode array uniform but this leads to increase in the power consumption. If the number of beams is made extremely large any way, an extent of deviation of beams from optical axes of optical elements of an optical system becomes substantial and optical characteristics are degraded.
On the other hand, as shown in FIG. 13, a surface emitting laser (VCSEL: Vertical Cavity Surface Emitting Laser) is a semiconductor laser that emits light in a vertical direction with respect to a substrate. This means that two-dimensional integration is straightforward. The electrical power consumed is in the order of one decimal place smaller compared to an end surface type laser. A larger number of light sources can therefore be integrated two-dimensionally.
A vertical cavity surface emitting laser that emits light vertically with respect to a semiconductor substrate surface has the following advantages compared to end surface emitting lasers of the related art. The volume of the active layer can be made small. Driving at a current of a low threshold value and low power consumption is therefore possible. The mode volume of an oscillator is also small so that modulation of a few tens of GHz becomes possible, which makes high speeds possible. An angle of spread of emitted light is also small and connection with optical fibers is therefore straightforward. Surface emitting lasers also do not require narrow openings to be manufactured. The surface area of the elements is therefore small. It is therefore possible to make a parallel, two-dimensional high-density array.
Examples of writing optical systems that employ a polygon mirror to perform scanning are given in Japanese Patent Application Laid-open No. 2004-287292 and Japanese Patent Application Laid-open No. 2008-107554.
Two-dimensional arraying of surface emitting laser diodes is straightforward and it is possible to increase the number of beams compared to end emitting laser diodes.
On the other hand, achieving a high output with a surface emitting laser diode is difficult. Moreover, when the spacing between surface emitting laser elements is too narrow, the lifespan of a light source is dramatically shortened due to thermal interference. In addition, arrangement of electrical wiring also becomes difficult when the spacing between surface emitting laser elements becomes too narrow. Methods that lower an absolute value for sub-scanning lateral magnification of an entire optical system with respect to a sub-scanning direction exist for broadening the spacing between elements of a surface emitting laser. However, conversely, when the absolute value for sub-scanning lateral magnification is lowered, optical utilization efficiency is also lowered and it is therefore necessary to increase the output of the light source. This is not an effective way of improving the lifespan of the light source.
The following is an explanation of a reciprocity law failure that occurs when writing at high-density.
Typical image forming units and apparatus such as copiers, printers, facsimiles, or multifunction products that are combinations thereof form images on an image carrier using the following means.
First, a region corresponding to an image pattern on an image carrier surface electrostatically charged by a charging unit such as a corona charger or a charging roller is irradiated with a light beam so as to form a latent image on the surface. Toner is then electrostatically affixed to the latent image by an exposure unit so as to form a toner image.
Forming of the latent image on the image carrier carried out here utilizes the characteristics whereby a latent image is formed when a charge load is attenuated as a result of a charged photoconductor being exposed to light so as to generate carriers within the image carrier. A PIDC (Photo-Induced Decay Curve) indicating an extent of attenuation of charge potential with respect to exposure energy shows a characteristic for each photoconductor. FIG. 14 explains an example of a PIDC.
The PIDC is decided for every photoconductor but the potential of the surface of the photoconductor after irradiation with a light beam is different depending on the way irradiation takes place even in the case of irradiation with a light beam of the same quantity of optical energy.
For example, with a mutual difference between a fall in potential of a photoconductor surface when the surface of the photoconductor is irradiated just once with a light beam having an optical energy of a certain quantity and a fall in potential of a photoconductor surface when irradiated twice with a light beam of an optical energy half of the aforementioned optical energy, an absolute value for the potential of the surface of the photoconductor falls substantially.
This is a typical phenomenon known in the related art as reciprocity. This phenomenon is discussed in Japanese Patent Application Laid-open No. 2003-205642.
This reciprocity law failure can be seen in image forming methods and apparatus that employ multi-beam scanning exposure methods. In a multi-beam scanning exposure method, when a plurality of laser diode light sources are lined up, the number of which is taken to be N, N-multi-beam lines are simultaneously exposed on the photoconductor when exposure in a main scanning direction is carried out one time using one surface of a rotating polygon mirror.
Each beam is elliptical and the adjacent beams partially overlap with each other. This means that a greater power than normal can be achieved with one exposure. When N multi-beam lines are then scanned to expose the next surface of the polygon mirror, scanning and exposure is such that beams for a final line one previous (Nth) and a first line (1st) on this occasion partially overlap. Exposure then takes place with this strong power being separated into two times.
In practical terms, in a multi-beam optical system, there are therefore cases where the same exposure imaging is incurred one time at one point on the photoconductor, and cases where the exposure energy is incurred divided into two times, even when the exposure energy provided to the photoconductor is the same. It is also possible for the effects to be different depending on the photoconductor at this time even when the same exposure energy is incurred. This is to say that the so-called reciprocity law failure occurs.
A PIDC for this time is shown in FIG. 15. A PIDC is shown in FIG. 15 where solid lines show the case where exposure takes place with the same exposure energy separated into two times (“sequential exposure” in the following), and dashed lines show the case of exposure one time (“simultaneous exposure” in the following).
When the images of dots (or lines) are formed using a plurality of beams, it can be understood that image defects referred to as “image irregularities” where the density and thickness of dots (or lines) that are formed by simultaneous scanning and exposure or sequential exposure of a plurality of beams change for a photoconductor of strong reciprocity.
It is also possible that density variation referred to as banding or a Moire pattern will be recognizable by a person depending on the size of the frequency of the image irregularities.
An explanation is now given of visual perception characteristics of a person.
When the image data is considered as the drawing of the distribution of density and brightness etc., this distribution can be perceived as a wave. Considering a pattern (wave) where light and dark are alternately repeated, it is possible to consider this repeating as a frequency. This frequency is typically referred to as spatial frequency.
When the spatial frequency is high, the width of the light and dark pattern becomes narrow. An MTF (Modulation Transfer Function) exists for denoting the relationship between spatial frequency and contrast (LD (Light-Dark) ratio). Visual perception characteristics of a person where spatial frequency f and contrast sensitivity MTF of a visual perception system are put into graphic form is shown in FIG. 16.VTF(u)=5.05 exp(−0.138u){1−exp(−0.1u)}  (1)                where u is spatial frequency [cycle/degree]        
It is necessary to process this image taking into consideration the characteristic that “perception becomes difficult towards the high-frequency side, but recognition becomes easier towards the low-frequency side”.
When variation in the sub-scanning frequency exists in regions that are perceptible to the human eye, when image forming is carried out using a multi-beam scanning method employing a plurality of beams to achieve high image quality and high speeds, there is the fear of writing being recognized as a direct Moire pattern.